Sometimes after booting, the network interface isn't up

Sometimes after booting, the network interface did not come up. I have to fix it manually then, issuing the command

/etc/init.d/networking restart

This brings the interface up, but also issues warning messages which perhaps may help you in tracking the problem:
This bug did not occur on Ubuntu 6.06, it started to appear when I upgraded to 6.10

------------- see below for the output of "networking restart" -----------------

There is already a pid file /var/run/dhclient.eth0.pid with pid 134993416
Internet Systems Consortium DHCP Client V3.0.4
Copyright 2004-2006 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

Listening on LPF/eth0/00:09:5b:05:85:f2
Sending on LPF/eth0/00:09:5b:05:85:f2
Sending on Socket/fallback
DHCPRELEASE on eth0 to 192.168.1.1 port 67
send_packet: Network is unreachable
send_packet: please consult README file regarding broadcast address.
There is already a pid file /var/run/dhclient.eth0.pid with pid 134993416
Internet Systems Consortium DHCP Client V3.0.4
Copyright 2004-2006 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

Listening on LPF/eth0/00:09:5b:05:85:f2
Sending on LPF/eth0/00:09:5b:05:85:f2
Sending on Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 5
DHCPOFFER from 192.168.1.1
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPACK from 192.168.1.1
bound to 192.168.1.36 -- renewal in 118248 seconds.
There is already a pid file /var/run/dhclient.eth1.pid with pid 134993416
Internet Systems Consortium DHCP Client V3.0.4
Copyright 2004-2006 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

SIOCSIFADDR: No such device
eth1: ERROR while getting interface flags: No such device
eth1: ERROR while getting interface flags: No such device
Bind socket to interface: No such device
There is already a pid file /var/run/dhclient.eth2.pid with pid 134993416
Internet Systems Consortium DHCP Client V3.0.4
Copyright 2004-2006 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

SIOCSIFADDR: No such device
eth2: ERROR while getting interface flags: No such device
eth2: ERROR while getting interface flags: No such device
Bind socket to interface: No such device
There is already a pid file /var/run/dhclient.ath0.pid with pid 134993416
Internet Systems Consortium DHCP Client V3.0.4
Copyright 2004-2006 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

SIOCSIFADDR: No such device
ath0: ERROR while getting interface flags: No such device
ath0: ERROR while getting interface flags: No such device
Bind socket to interface: No such device
There is already a pid file /var/run/dhclient.wlan0.pid with pid 134993416
Internet Systems Consortium DHCP Client V3.0.4
Copyright 2004-2006 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

SIOCSIFADDR: No such device
wlan0: ERROR while getting interface flags: No such device
wlan0: ERROR while getting interface flags: No such device
Bind socket to interface: No such device